The centre of each board is raised higher than the edges giving the appearance of a series of small mounds with a convex shape.
If the unfinished surface of the flooring boards has been exposed to moisture prior to finishing, the flooring can become crowned due to expansion of the wood surface. In practice, this rarely occurs.
Sometimes on flat sawn flooring boards (as opposed to quarter sawn), there can be differences in tangential and radial moisture movement, which, when there is a change in the moisture content of the flooring, can cause crowning or cupping. As the flooring dries, the movement occurs away from the pith (the young wood at the centre of the heartwood) and as moisture increases the movement occurs toward the pith.
Another possibility is that a previously cupped floor has been sanded down before being allowed to dry properly, making the edges thinner. Consequently, if flooring does then dry out accordingly the bottom of the boards shrink and flatten causing the edges of the board to be lower than the centre
A) Underside of flooring gains moisture from excessively humid sub-floor, or from spillage. The underside expands and leads to cupping.
B) The floor is sanded flat before drying has completed. The surface is flat but the underside is still cupped.
C) The underside of the flooring continues to dry and flatten and the surface becomes crowned (i.e. convex).
Drill two small holes through the flooring and using a moisture metre take readings from the sub-floor in order to ascertain whether there is excess moisture. Also take readings from a cross section of the flooring to see if there is a moisture content greater than expected.
Where flooring has become crowned due to the sanding flat of cupped floor that has then dried out, sanding this flat again can provide the solution. Ensure that the bonding to the sub-floor, whether by flooring adhesive or secret nailing is still secure and consistent. Vertical movement and or creaking may be an indication of flawed sub-floor bonding. Loose boards can usually be fixed by drilling, face nailing then plugging or filling.
Alternatively, if the flooring has been installed on a timber based, load bearing sub-floor that can be accessed from below such as, from a basement. Drill a pilot hole through the timber sub-floor and into the wood flooring itself and screw into the two layers to tighten down the flooring.